Inactivation of vancomycin-induced unstable L-forms of Staphylococcus aureus by horse serum

Substitution at codon Ser۳۱۵ of katG gene, a reliable marker for isoniazid (INH) resistance and proliferate in spite of the loss of cell wall is called an L- form. Unstable L- forms  can convert to normal cells but stble L-forms can not.cell wall - deffective state may be induced spontaneously or through the action of an appropriate agent such as antibiotic interfering with the synthesis of cell wall peptidoglycan. Vancomycin is one of these antibiotics which cause L-form formation in some strains of Staphylococcus aureus in presence of osmoprotective compounds. Horse serum and some of organic or mineral materials can be used for better isolation of L- forms in vitro. In this study, standard strain of  Staphylococcus aureus (ATCC ۲۵۹۲۳) was inoculated  to ۲ media: i) L phase medium (LPM) agar containing horse serum & ii) LPM agar without horse serum. Sucrose was used in both above mentioned growth media as osmoprotective and vancomycin was used for inducing L-forms. Microscopic examination revealed presence of small and large colonies on LPM agar lacking horse serum and only large colonies on LPM agar containing horse serum after ۳ - ۴ days. induced by horse serum were unable to form colony in LPM agar containing horse serum. Results showed that unstable L-forms of Staphylococcus aureus in the presence of horse serum neither can survive nor convert to normal cells.